"Keep it secret. Keep it safe!" hobbit Frodo is told by wizard Gandalf in the first part of Peter Jackson's Lord of the Rings trilogy, 2001's The Fellowship of the Ring. It's advice that Martin Freeman, star of forthcoming prequel The Hobbit, might wish he had taken heed of after he blunderingly revealed to reporters that his Sherlock Holmes co-star Benedict Cumberbatch has taken a secret role in the new two-part film.

Freeman, who plays Watson to Cumberbatch's Holmes on the BBC television series, dropped the clanger at the Bafta TV awards at the weekend. After making the admission to reporters in the press room, he said: "I knew I was a big mouth, I didn't know how much of a big mouth, and I've ruined everything. And now I might not have a job to go back to!"

If the casting is confirmed and Cumberbatch makes the trip to New Zealand, where filming is currently taking place, he will join a cavalcade of UK talent on the project. As well as Freeman (set to play Bilbo Baggins), Jackson has cast Stephen Fry (as the Master of Laketown), Ian McKellen, Andy Serkis (returning as Gandalf and Gollum), Spooks' Richard Armitage (dwarf leader Thorin Oakenshield) and Sylvester McCoy (wizard Radagast the Brown).

The Hobbit's screenplay is by Guillermo del Toro, Jackson and his regular writing team of Fran Walsh and Philippa Boyens. The two-part film will be shot back-to-back and released in December 2012 and 2013.